Over recent years, with the popularization of car-interior displays, large-screen TV sets, mobile phones, and lap-top computers, there is an increasing demand for liquid crystal displays (hereinafter also referred to LCDs) which serve in a category of various display devices. LCDs have been widely used as monitors due to their small footprint and low energy consumption features, compared to old-fashioned CRT displays, and have become common in application to TV sets. For these LCDs, various optical films such as polarization films or retardation films are used.
Such an LCD is structured in such a manner that polarization plates are provided on both sides of a liquid crystal cell. A polarization plate passes only light of a polarized wave plane from a predetermined direction. Therefore, the polarization plate plays a significant role in visualizing variations of the orientation of a liquid crystal via an electric field in a liquid crystal display. Namely, performance of the liquid crystal display largely depends on performance of the polarization plate. The polarization plate is commonly structured in such a manner that protective films are laminated on both sides of a polarizer. In some cases, such protective films have a retardation compensation function. LCDs are structured by laminating thus-structured polarization plates to a liquid crystal cell. Protective films are provided to enhance durability of a polarizer. Conventionally, as protective films used for polarization plates, optical films, which are transparent and exhibit excellent physical and mechanical properties, as well as minimal dimensional variation against varying temperature or humidity, have been used.
Recently, with the increasing demand for various display devices, productivity enhancement has been demanded for optical films for use in these devices. In order to increase productivity of optical films, width increase of the optical films and a high-rate production process are needed. Further, to make various display devices thinner, thinner and lighter optical films have been sought. Still further, with realization of larger screen sizes of various display devices, width increase of optical films has also been demanded. Further, for higher productivity, winding of longer films on a single core has been in progress.
Further, to improve mainly mechanical strength of these optical films, and also film characteristics, storage stability, and optical characteristics, it is necessary to add, to the optical films, various additives (e.g., a plasticizer, an antioxidant, a UV absorbent, a matting agent, a conductive substance, an antistatic agent, a flame retardant, and a lubricant).
Conventionally, such optical films have been produced via a solution casting film forming method wherein a dope, prepared by dissolving a resin and various additives in a solvent, is cast on an endless support, and then the solvent is removed in a drying process to wind the film. When optical films are produced via the solution casting film forming method, optical characteristics and flatness thereof may be adjusted by stretching employing a tenter after film formation.
As a matter of fact, inclusion of the aforementioned additives in the film may result in bleed out of the additives depending on the environmental conditions such as temperature and humidity when the optical film is stored for a prolonged period. This bleed out caused problems of decrease of product quality and decrease of productivity due to the deterioration in dimensional stability, storage stability or optical properties.
There have been investigated various methods to prevent bleed out of the aforementioned additives. For example, there was disclosed a method to use a hydrogenated petroleum resin as a plasticizer in a cellulose ester film to prevent bleed out (e.g., refer to Patent Document 1).
Another known example is a method to use an isocyanate cross-linking agent in a solvent casting process for producing a cellulose ester film. This method will result in producing a cellulose ester optical compensation film without a defect of bleed out of additives such as a plasticizer (e.g., refer to Patent Document 2).
Further known example is a method to use a silane coupling agent as an additive. This will prevent bleed out of plasticizers and UV absorbers (e.g., refer to Patent Document 3).
The methods described in Patent Documents 1 to 3 are, in fact, efficient ways to prevent plasticizers and UV absorbers. However, they are not sufficient to prevent bleed out of additives over time (during storage) incorporated in an optical film having a decreased thickness and an increased width and wound in a winding core to form a long roll so as to increase productivity of the film, which is recently developed. The film roll made by this production method may result in bending of the roll in the center portion due to the weight of itself. When the film roll is stored over a period of time under a stressed condition in the center portion of the roll, bleed out of additives tends to occur. In addition, these proposed methods each use a specific type of plasticizer, cross-linking agent and coupling agent. Therefore, they may lack versatility and may possibly increase a production cost.
Under these circumstances, it has been required to produce an optical film, a production method thereof and a polarization plate free from bleed out by incorporating a plasticizer or an additive of general use and in a form of a long roll having a decreased thickness and an increased width even when it is stored over time (during storage).
Patent Document 1: Unexamined Japanese Patent Application Publication (hereinafter also referred to as JP-A) No. 2003-96237
Patent Document 2: JP-A No. 2006-71876
Patent Document 3: JP-A No. 2006-131737